THE KIVER MUSSEL. 153 



longitudinal fibers of the oblique pedal muscles, capable 

 of retracting it, or of moving it sidewise, and of circular 

 fibers capable of extending it. It is adapted for plowing 

 through mud or sand, and for no other kind of locomotion. 

 Only by sinking the foot into mud or sand is the animal 

 able to rise to its erect position. Blood is forced down 

 the pedal artery through the center of the foot into its 

 point, as an additional aid to its extension; and by the 

 distension of its tip a firmer hold is obtained in the sand, 

 to aid in locomotion. The flat muscle of the mantle is 

 capable of moving its entire margin. 



IV. Sensation. The nervous system of the mussel 

 consists of the three pairs of ganglia already noticed, and 

 of nerves radiating from each pair extending to every 

 vascular part of the body, giving sensibility to. every part, 

 and furnishing nervous communication with every other 

 part. 



The senses are not very acute. Touch is well devel- 

 oped, and is most acute in the point of the foot and in the 

 mantle margin, particularly in the part which forms the 

 outer border of the siphons. Taste and smell are prob- 

 ably developed slightly, and aid the animal somewhat in 

 the selection of food. Sight and hearing are developed 

 but feebly, if at all. There is a so-called ear sac in 

 the foot, near the pedal ganglia. That it is an organ of 

 hearing is very doubtful. It seems more probable that it 

 is an organ of the sense of equilibrium, enabling the ani- 

 mal to distinguish between a flat or inclined position and 

 an erect position. Though incapable of hearing sound, 

 the animal may feel vibrations when they become suf- 

 ficiently intense. The siphon fringes are so sensitive to 

 light, that a heavy shadow thrown across them will cause 

 them to be retracted. 



The instincts of this animal are of a low order : they 

 seem to be limited to self-preservation and the selection 



